a. Chondrichthyes and ostichthyes

Chodrichthyes :

• Their habitat is mainly marine and their body is streamlined. They have cartilaginous endoskeleton.

• Mouth is ventrally (near the front of the body) located.

• Notochord is persistent throughout the life.

• The air bladder is absent due to which they have to swim constantly to prevent sinking.

• They have tough skin, containing placoid scales. The teeth are modified placoid scales directed backwards.

• Heart is 2-chambered and they are cold blooded.

• Gill slits are separated and without operculum (cover for gills)

• Examples – Scoliodon, Pristis , Trygon

Ostichthyes

• The habitat is both marine as well as freshwater and they have a streamlined body. The endoskeleton is bony.

• Mouth is mostly terminal.

• Air bladder is present which gives buoyancy.

• There are 4 pairs of gills with operculum present.

• They are cold blooded.

• Heart has 2 chambers (1 auricle and 1 ventricle).

• Skin is covered with cycloid/ctenoid scales.

• Examples – Marinee : Hippocampus (Sea horse), Freshwater : Labeo (Rohu), Aquarium : Betta (Fighting fish)

b) Urochordata and cephalochordata

• Phylum chordata is divided into three categories – urochordata, cephalochordata and vertebrate.

• Urchordata and cephalochordate are often referred to as protochordates.

• They are exclusively marine.

• In urochordates, the notochord is present only in larval tail but in cephalochordates, notochord extends from head to tail region and is persistent throughout the life.

• Examples – Urochordates: Ascidia , Salpa , Cephalochordates : Branchiostoma.